The OFDM transmission system is adopted for the digital television broadcast, the wireless LAN (Local Area Network), and the like. The OFDM transmission system is a kind of the multi-carrier transmission system. According to the OFDM transmission system, pieces of digital data are respectively assigned to a plurality of carriers, the carriers are modulated with the pieces of digital data, and then the modulated carriers are multiplexed and transmitted.
Since the OFDM transmission system can extend a time period of each modulation symbol by using a plurality of carriers, the OFDM transmission system is known as transmission system resistant to the multipath interruption.
Also, according to the OFDM transmission system, a signal to be transmitted is generally composed of a period necessary for actually transmitting a signal that relates to data, which is called a useful symbol period, and a redundant period for transmitting a signal that is the same as part of the signal transmitted in the useful symbol period, which is called a guard interval period. The signal transmitted in the guard interval period is also called a cyclic prefix, and prevents, against a delay wave due to the multipath propagation, the inter-symbol interference, and the inter-carrier interference by maintaining the inter-carrier orthogonality.
Note that a signal transmitted in accordance with the OFDM transmission system is called “OFDM signal”, and the OFDM signal includes a signal transmitted in a useful symbol period and a signal transmitted in a guard interval period.
Conventionally, there have been known a frequency synchronization method and a symbol synchronization method. According to these methods, by utilizing that a signal transmitted in a guard interval period and part of a signal transmitted in a useful symbol period have the same signal waveform, frequency synchronization of carrier frequency and symbol synchronization for identifying a symbol are performed based on a correlation between the signal transmitted in the guard interval period and the part of the signal transmitted in the useful symbol period. However, if an OFDM signal on which a narrow-band interference wave has been superposed is received, the correlation is influenced by the narrow-band interference wave, and there occurs an error in the frequency synchronization and the symbol synchronization. This is due to the following. The narrow-band interference wave has a strong time correlation. When a correlation is calculated between a signal transmitted in a guard interval period and part of a signal transmitted in a useful symbol period, the correlation of the narrow-band interference wave is superposed on the correlation between the signals, as an error in additivity.
Note that the narrow-band interference wave indicates an interference wave having a frequency bandwidth narrower than a frequency bandwidth of a signal transmitted in accordance with the OFDM transmission system.
The Patent Document 1 discloses an OFDM receiver that solves the above problem. The following describes the conventional OFDM receiver disclosed in the Patent Document 1 with reference to the drawing.
According to a conventional OFDM receiver 100 shown in FIG. 11, OFDM signals received by an antenna 111 are input to a tuner 112. The tuner 112 selects an OFDM signal of a desired channel among the OFDM signals, and then converts the selected OFDM signal into an OFDM signal of an IF (Intermediate Frequency) band. The tuner 112 outputs the OFDM signal of the IF band to an analog to digital convertor (hereinafter, “A/D convertor”) 113. The A/D convertor 113 converts the OFDM signal from an analog signal into a digital signal, and outputs the digital OFDM signal to an IQ demodulation circuit 114, which is a quadrature detector circuit. The IQ demodulation circuit 114 performs quasi-synchronous quadrature detection on the digital OFDM signal so as to be converted into a complex baseband signal. Note that the frequency control circuit 124 performs control such that the detection frequency of the detection signal used by the IQ demodulation circuit 114 for performing quasi-synchronous quadrature detection synchronizes with a frequency of an OFDM signal input to the IQ demodulation circuit 114.
The IQ demodulation circuit 114 outputs the complex baseband signal (hereinafter, “IQ demodulated signal”) to an FFT circuit 115. The FFT circuit 115 detects a useful symbol period of the IQ demodulated signal based on a later-described detection signal input from a timing detection circuit 120. Then, the FFT circuit 115 performs an FFT (Fast Fourier Transform) calculation on the IQ demodulated signal corresponding to the detected useful symbol period, and converts the IQ demodulated signal from data on a time axis into data on a frequency axis. A demodulation circuit 116 demodulates the data obtained as a result of the FFT calculation performed by the FFT circuit 115, and an error correction circuit 117 performs error correction processing on the demodulated data.
Also, the complex baseband signal (IQ demodulated signal) output from the IQ demodulation circuit 114 is input to a correlation detection circuit 118. The output IQ demodulated signal is delayed by a useful symbol period delay circuit 119 by a useful symbol period, and then is output to the correlation detection circuit 118. The correlation detection circuit 118 detects a correlation of a guard interval period between the IQ demodulated signal and the IQ demodulated signal which has been delayed by the useful symbol period (hereinafter, “useful symbol period delay signal”). Here, the correlation detection operations performed by the correlation detection circuit 118 are described with reference to FIG. 12(a) to FIG. 12(d).
FIG. 12(a) shows an IQ demodulated signal output from the IQ demodulation circuit 114, and FIG. 12(b) shows a useful symbol period delay signal output from the useful symbol period delay circuit 119. Note that one symbol period of the IQ demodulated signal is composed of a guard interval period and a useful symbol period for transmitting a signal relating to data. The guard interval period has a copy of the latter part of the useful symbol period.
The correlation detection circuit 118 multiplies an IQ demodulated signal by a complex conjugate of a useful symbol period delay signal to calculate an I component and a Q component of a correlation value of the IQ demodulated signal and the useful symbol period delay signal. The correlation value calculated by the correlation detection circuit 118 is greater in a guard interval period of the useful symbol period delay signal whose IQ demodulated signal and useful symbol period delay signal match each other, as shown in FIG. 12(c). Note that FIG. 12(c) shows an I component of a correlation value, and does not show a Q component of the correlation value.
Note that the “I component” and the “Q component” respectively indicate an “inphase component” and a “quadrature component” in the present Specification.
The correlation detection circuit 118 calculates a moving average of each of the I component and the Q component of the correlation value using a width of a guard interval period, and outputs a correlation signal whose I component and Q component are respectively I component and Q component of the moving average values. As shown in FIG. 12(d), a peak of the correlation signal is at a head of a useful symbol period of a useful symbol period delay signal. FIG. 12(d) shows the I component of the correlation signal, and the Q component of the correlation signal is not shown in the figure. Note that if there is no frequency error between a frequency of an OFDM signal input to the IQ demodulation circuit 114 and a detection frequency of a detection signal to be used by the IQ demodulation circuit 114 for performing quasi-synchronous quadrature detection, the I component of the correlation signal has a peak, and the Q component of the correlation signal is substantially 0.
The timing detection circuit 120 detects a timing showing a head of a useful symbol period of an IQ demodulated signal based on an input correlation signal, and outputs a detection signal based on a result of the detection to the FFT circuit 115.
An offset detection circuit 121 and a correction circuit 122 cancel a component caused by a narrow-band interference wave from an I component and a Q component of a correlation signal of a guard interval period of a useful symbol period delay signal to correct the I component and the Q component of the correlation signal, as described later.
A tan−1 circuit 123 detects a guard interval period of the useful symbol period delay signal based on the detection signal input from the timing detection circuit 120, and calculates a phase angle of the correlation signal using the I component and the Q component of the correlation signal of the detected guard interval period. Then, the frequency control circuit 124 controls a detection frequency of a detection signal to be used by the IQ demodulation circuit 114 for performing quasi-synchronous quadrature detection so as to synchronize frequencies, based on an error signal indicating a value of the phase angle of the correlation signal input from the tan−1 circuit 123.
The following describes the operations of the OFDM receiver 100 for receiving an OFDM signal on which a CW (Continuous Wave) interference wave that is a kind of interference wave is superposed, with reference to FIG. 13(a) to FIG. 13(g).
FIG. 13(a) shows an IQ demodulated signal output from the IQ demodulation circuit 114. FIG. 13(b) shows a useful symbol period delay signal output from the useful symbol period delay circuit 119. Note that the IQ demodulated signal shown in FIG. 13(a) and the useful symbol period delay signal shown in FIG. 13(b) are each a signal on which a CW interference wave component relating to a CW interference wave is superposed.
The correlation detection circuit 118 calculates a correlation value between the IQ demodulated signal and the useful symbol period delay signal. The correlation value calculated by the correlation detection circuit 118 is a value different by a certain amount from a correlation value relating to an OFDM signal on which no CW interference wave is superposed, as shown in FIG. 13(c). Note that FIG. 13(c) shows an I component of a correlation value, and a Q component of the correlation value is not shown in the figure.
The correlation detection circuit 118 calculates a moving average of each of the I component and the Q component of the correlation value using a width of a guard interval period, and outputs a correlation signal whose I component and Q component are respectively an I component and a Q component of the moving average values. The correlation signal output from the correlation detection circuit 118 is a signal different by a certain amount from a correlation signal relating to an OFDM signal on which no CW interference wave is superposed, as shown in FIG. 13(d). Note that FIG. 13(d) shows an I component of a correlation value, and a Q component of the correlation value is not shown in the figure.
The timing detection circuit 118 inputs a timing signal shown in FIG. 13(e) indicating a predetermined period T other than the guard interval period to the offset detection circuit 121. The offset detection circuit 121 calculates an average value in the predetermined period T of each of the I component and the Q component of the correlation signal input from the correlation detection circuit 118 (hereinafter, “offset amount”). Then, the offset detection circuit 121 outputs a signal as shown in FIG. 13(f) indicating an offset amount of each of the I component and the Q component to the correction circuit 122.
Based on a timing signal shown in FIG. 13(g), the correction circuit 122 subtracts, from the I component and the Q component of the correlation signal input from the correlation detection circuit 118 corresponding to the guard interval period of the useful symbol period delay signal, the offset amount of each of the I component and the Q component input from the offset detection circuit 121, and outputs a value obtained as a result of the subtraction of each of the I component and the Q component of the correlation signal to the tan−1 circuit 123.
[Patent Document 1] Japanese Laid-Open Patent Application Publication No. 2002-290371